BDNF and depressive symptom measurement: The increase in BDNF and related synaptogenic factors are considered essential to ketamine’s antidepressant effects (Li et al., 2011; Duman et al., 2016), and BDNF has been identified as a potential candidate as a biomarker of depression severity and treatment response (Sen et al., 2008; Bocchio-Chiavetto et al., 2010).